Paper making stock requires screening before use on a paper making machine to remove undesirable particles (reject) which would otherwise show up as defects in the paper sheet, and which are too close in specific gravity to good paper making fibers for removal by centrifugal cleaning. These undesirable particles can conveniently be classified into two broad groups: (a) particles of elongated but thin shapes, such particularly as shives, splinters or slivers of wood, pieces of yarn or string, and flakes or strips of wet strength paper and plastic sheet or film, and (b) relatively chunky particles, such as bits of rubber or foamed plastic, fragments of bark, and system debris.
The particular types of these two groups of reject particles vary depending on the nature of the paper making stock. Thus in the case of virgin pulp at the beater room stage, the most prevalent of the group (a) particles would usually be shives or splinters, while the group (b) particles would usually be bits of bark. In waste paper furnishes, there are unlikely to be any significant quantity of shives at the beater room stage, but there will be substantial amounts of string, wet strength paper and plastic sheet and film, while the group (b) particles may be of any of a wide variety of contaminates, particularly bits of foamed plastic, rubber and hard plastic.
All types of furnishes are also subject to accumulation of system debris between the beater room and the paper machine, such for example as bits of the system itself (pipe-lining material, scale, etc.) and material which falls into a chest or other open part of the system, such as hard plastic safety hats which are gradually reduced to small particles in a pump.
It has become increasingly important in paper making to screen the stock immediately ahead of the head box as the paper industry has in recent years tended toward the use of increasingly narrow slice openings from the head box. This is especially true in pressure formers on tissue machines wherein extended slice passages as narrow as 0.03 inch are not uncommon, and any chunky particles having one or more dimensions greater than the slice width are likely to jam therein and cause disruption of the stock flow onto the breast roll. This problem is also accentuated by the increasing use of substantially higher consistency stock at the slice, namely consistencies as high as 4% solids.
It has accordingly been a common practice for some years in the paper industry to install one or more pressure screens immediately ahead of the head box, and this practice has become more common in response in the increased use of narrower slice openings. Until the present invention, however, there was no screen available to the industry which solved all the related problems.
More specifically, if the screening member of a pressure screen were provided with circular screening holes of sufficiently small diameter to reject particles significantly smaller than e.g. 0.062 inch in cross-section, it not only would reject a substantial quantity of good long fiber, but it would also have an impractically low capacity and unacceptably high power requirements. In addition, the smaller the holes in such a screen, the greater are the power requirements for the pump or pumps by which the stock is fed through it to the head box.
The use of previously available screens incorporating a slotted screen member has other disadvantages. Screens such as are disclosed in Seifert U.S. Pat. No. 3,849,302, wherein the slots extend axially of the screening cyclinder, are extremely effective for rejecting elongated thin contaminate particles, but they also tend to reject a substantial number of the long fibers. In fact, the effectiveness of such screens for fractionating fiber stock by length of fiber is disclosed and claimed in Seifert U.S. Pat. No. 3,909,400, and it is necessary to maintain the feed stock at low consistency, i.e. not substantially greater than 1%, to minimize the tendancy to fractionation, which increases markedly as the consistency is increased.
Slotted screens of the construction disclosed in our U.S. Pat. No. 4,155,841, wherein the slots extend circumferentially of the screening cylinder, do not have the fractionating effect of slotted screens wherein the slots extend axially of the screening cylinder, and they are highly effective for fine screening purposes when they follow a screen which removes the elongated thin contaminate particles. Such screens, however, wherein the maximum width of the slots is disclosed and claimed as 0.008 inch, are of relatively low capacity under economical conditions of power consumption, and they have not been proved capable of effectively handling stock of higher consistency than the slotted screens of the Seifert patent. Their use immediately ahead of a paper machine would therefore be impractical.